This invention relates to a circuit for use in an electric shutter which is operated to feed exposure information such as ASA sensitivity or aperture information, except brightness information.
As a method for electrically processing the exposure information, it has been proposed to set a resistance value to correspond to each element of exposure information, such as ASA sensitivity or aperture setting and to develop electrical signals using the set resistance values for applying exposure information electrically. However, in a shutter of the type having a low brightness sensor circuit, except a time constant circuit, for use in controlling the shutter, the exposure information should be simultaneously and efficiently fed to these two circuits. The exposure time t will be expressed as follows: ##EQU1## where, R: a resistance value of the photoelectric element
C: an electrostatic capacitance of the capacitor PA1 E: a voltage of the power supply PA1 V: a reference voltage PA1 E: a power supply voltage PA1 V: a reference voltage
The value of t is used in the case that an integrating circuit comprised of the photoelectric element and the capacitor is used as a shutter control time constant circuit. To the contrary, in the case that a photoelectric element and a comparison resistance are connected in series to form a low brightness sensor circuit, and its partial voltage is compared with a reference voltage, a value of the resistance R of the photoelectric element when the reference voltage corresponds with the output voltage indicating the brightness is as follows: ##EQU2## where, R.sub.O : a resistance value of the comparison resistance
For example, when ASA sensitivity exposure information is fed as a reference voltage V, this reference voltage V should be set such that the exposure time t assumes the values 2t, t, 1/2t, 1/4t . . . as ASA sensitivity is varied among the corresponding values 50, 100, 200, 400 . . . To the contrary, in a low brightness sensor circuit, the reference voltage V should be set such that the resistance R of the photoelectric element assumes the value 1/2R, R, 2R, 4R . . . in response to the variations of ASA sensitivity as described above. However, as is apparent from equations (1) and (2) above, the equation describing the time constant circuit is a logarithmic function and the equation describing the low brightness sensor circuit is a fractional function. Therefore, it is not possible to construct the two circuits described above by using the same reference voltages V.